I would not use cornerstones for sashing when both the cornerstones and sashing are the same fabric. It's unnecessary and, in my opinion, doesn't add anything to the quilt.
For lining up, what I do is sew all the short sashings on first, creating rows of blocks. Then I sew the long sashing on to the bottom of the first row. Before continuing, I turn this row over and use a pencil and ruler to mark the seam lines all the way to the remaining raw edge of the long sashing strip. When attaching the second row, I pin first, lining up the seams on the second row with the marks on the first row's sashing. When sewing, ease or stretch as necessary to get the marks and seams to match. This will ensure that all the blocks will line up when you are finished. It is the same concept as using cornerstones, but without the extra sewing.